One of the most common sources of error is students rounding off their calculator display before approaching their final answer, and doing so incorrectly. Although this seems relatively straightforward, a mark may still be lost, as half marks are never awarded in mathematics.
Students should pay careful attention to how the exam specifies to display their final answers; such as to 2 decimal places (d.p), 2 significant figures, nearest dollar, nearest km, and so forth.
To avoid making reckless oversights in an exam, always UNDERLINE what they specify in the question.
But What if the Question Doesn’t Specify?
If the question doesn’t tell you how to round, the rule of thumb is to round to the LEAST number of significant figures (s.f.) given in the question. We can break this down further into two rules.
- The final result of multiplication or division should have no more digits than the numerical value with the fewest significant figures.
- When adding or subtracting numbers, the final result should contain no more decimal places than the number with the fewest.
Although the example below is from chemistry, the same principle applies across MATH and SCIENCE.
Ignore the math. Look at our I wrote my answer to 3 s.f. to match the least s.f. in my question!
How to Get Around Not Rounding Until the End
SHIFT –> STO –> A
Say that again in your head. These are steps to storing a value in your CASIO calculator, the standard HSC calculator in NSW. It doesn’t need to be A, it can be B, C, D, E, F, X, Y, M. When you want to summon the stored value, hit
ALPHA –> A
Done. Now instead of having to worry about rounding too early in a question, you can use the EXACT input values you need in order to get your final rounded answer.
HINT ~ This is very useful in 3D TRIG, check it out.
Anthony, from Kalibre Education.
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